Technical Field
The present disclosure relates to an abnormality diagnosis apparatus applied to an exhaust gas purification apparatus of an internal combustion engine.
Description of the Related Art
It is known in prior arts to provide a NOx storage reduction catalyst (which will also be referred to as the “NSR catalyst” hereinafter) in an exhaust passage of an internal combustion engine that operates in a lean burn mode, in which the air-fuel ratio of the air-fuel mixture is a lean air-fuel ratio higher than the stoichiometric air-fuel ratio. The NSR catalyst has the function of storing NOx in the exhaust gas when the atmosphere around it has a lean air-fuel ratio and reducing NOx stored therein under the presence of a reducing agent when the atmosphere around it has a rich air-fuel ratio lower than the stoichiometric air-fuel ratio. It should be noted that the term “store” also means to adsorb, in this specification.
Patent Literatures 1 to 4 in the list given below disclose technologies pertaining to the method of determining deterioration of an NSR catalyst. For example, according to Patent Literature 1, two oxygen sensors having different sensitivities are provided downstream of an NSR catalyst. The difference between these sensors in the time until the sensor output becomes equal to or smaller than a specific value is measured. Furthermore the NOx storage amount (i.e. the amount of NOx stored in the NSR catalyst) is estimated. If the time difference measured as above is smaller than a predetermined time, it is determined that the NSR catalyst is sulfur poisoned. If the time difference is equal to or larger than the predetermined time and the estimated amount of stored NOx is smaller than a predetermined reference amount, it is determined that the NSR catalyst is deteriorated by heat.
According to Patent Literature 2, an ideal NOx storage amount is calculated using a map that specifies the NOx storage amount in the NSR catalyst taking account of aging, and the actual NOx storage amount is calculated on the basis of a measurement value of an NOx sensor provided in the exhaust passage. If the difference between the ideal NOx storage amount and the actual NOx storage amount after performing desulfurization control is smaller than a threshold, it is determined that the NSR catalyst has recovered from sulfur poisoning by the desulfurization control. If the difference is larger than the threshold, it is determined that the NSR catalyst is deteriorated by heat.
According to Patent Literature 3, the NOx storage amount is calculated on the basis of an output of a sensor provided downstream of the NSR catalyst during what is called the rich spike process. If the NOx storage amount thus calculated is smaller than a threshold, it is determined that the NSR catalyst is deteriorated.
According to Patent Literature 4, the NOx storage amount in the NSR catalyst is estimated. If the concentration of NOx measured by an NOx sensor provided downstream of the NSR catalyst is equal to or higher than a specific concentration at the time when the estimated NOx storage amount reaches the maximum amount of NOx that the NSR catalyst in a normal condition can store, after storage of NOx by the NSR catalyst is restarted after NOx reduction control, it is determined that the NSR catalyst is deteriorated.